Allow for import/export of palette files from other design applications

[toolness]

There is some kind of format called .clr that tools like Adobe and Sketch use; we should totes support it!

[toolness]

Uhhh, so it seems like this is actually a proprietary Apple format that native apps have to call out to Cocoa APIs to manipulate. I guess we could wrap this app in Electron and link it to native code that does the CLR import/export, but that is definitely a fairly heavy lift.

[toolness]

Um, aside from that, there is no way to import .clr files in Sketch, at least.

But there is a sketch palettes plug-in that uses a nice JSON format. There's also the Adobe Swatch Exchange (ASE) format, which is documented.